1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a memory resistor cell made using an electrode with nanotips.
2. Description of the Related Art
Semiconductor IC memory devices have replaced magnetic-core memory devices, as IC devices have lower fabrications cost and exhibit higher performance. An IC memory circuit includes a repeated array of memory cells, each of which stores one state of a two-state information (0 or 1), or multi-state information (for example, 00, 01, 10, or 11 of 4 states). This array of cells works together with support circuitry such as a row decoder, a column decoder, a write circuit to write to the memory cell array, a control circuitry to select the correct memory cell, and a sense amplifier to amplify the signal.
One conventional memory circuit, the flip-flop, has an output that is stable for only one of two possible voltage levels. SRAM (static random access memory) circuits store information in flip-flops where the information can be read from any memory cell at random (random access memory), and where the stored information can be kept indefinitely as long as the circuit receives power.
The more recently developed memory cell is a DRAM (dynamic random access memory) cell. A DRAM cell typically includes a transistor and a capacitor. The capacitor stores information in the form of electrical charge and the transistor provides access to the capacitor. Because of the inherent leakage of the capacitor charge, DRAM cells must be rewritten or refreshed at frequent intervals.
SRAM and DRAM memories cannot retain the stored information without a power source. Therefore, they belong to a class of memory called volatile memory. Another class of memory is called non-volatile memory, which retains the stored information even after the power is turned off.
A typical non-volatile memory is ferroelectric random access memory (FRAM). Similar to a DRAM cell, a FRAM cell includes an access transistor and a storage capacitor. The difference is that FRAM cell uses ferroelectric material for its capacitor dielectric, and the information is stored as the polarization state of the ferroelectric material. Ferroelectric material can be polarized by an electric field with a polarization lifetime of longer than 10 years.
Recent developments in materials, with changeable electrical resistance, have introduced a new kind of non-volatile memory, called RRAM (resistive random access memory). The basic component of a RRAM cell is a variable resistor. The variable resistor can be programmed to have high resistance or low resistance (in two-state memory circuits), or any intermediate resistance value (in multi-state memory circuits). The different resistance values of the RRAM cell represent the information stored in the RRAM circuit. The advantages of RRAM are the simplicity of the circuit, resulting in smaller devices, non-volatile memory characteristics, and an inherently stable memory state.
Since a resistor is a passive component, and cannot actively influence nearby electrical components, a basic RRAM cell can formed with just a variable resistor, arranged in a cross point resistor network to form a cross point memory array. To prevent cross talk or a parasitic current path, a RRAM cell can further include a diode. This resistor/diode combination is sometimes called a 1R1D (or 1D1R) cross point memory cell. To provide better access, a RRAM can include an access transistor, as in a DRAM or FRAM cell, and this combination is sometimes called a 1R1T (or 1T1R) cross point memory cell.
The resistance states of the RRAM can be represented by different techniques such as structural, polarization, or magnetization state. A Chalcogenide alloy is an example of structural state RRAM device. Chalcogenide alloys can exhibit two different stable reversible structural phases, namely an amorphous phase with high electrical resistance, and a polycrystalline phase with lower electrical resistance. Resistive heating by an electrical current pulse can change the phases of the chalcogenide materials. One example of polarization state is a polymer memory element. The resistance state of a polymer memory element is dependent upon the orientation of polarization of the polymer molecules. The polarization of a polymer memory element can be written by applying an electric field.
Conventional memory resistor RRAM cells are made using planer metal top and bottom electrodes. The field intensity and the current density are typically very uniform across the electrode. Further, the physical structure of these devices is typically quite symmetrical. The symmetrical cell construction and uniform field intensities dictate that the memory states be changed using unipolar switching. That is, the resistor may be reversibly programmed to a high or a low resistance state by unipolar electrical pulses having different pulse widths. The power dissipation is, therefore, equal to IV. A relatively high current density is required for programming, which may be higher than the capacity of minimum-sized MOS transistors. For use in practical commercial applications, it would be desirable if the RRAM memory states could be switched using bipolar electrical pulses. One approach this problem has been to build physically asymmetric cells. Another approach has been to structure the memory resistor material, to create non-uniform field intensities in the memory resistor. However, these solutions may require extra fabrication processes.
It would be advantageous if a non-volatile resistor memory cell could be fabricated, that would be suitable for low-power, high-density, large-scale memory applications.
It would be advantageous if a non-volatile resistor memory cell could be practically fabricated using conventional CMOS processes, that could be programmed using bipolar pulses, as well as unipolar pulses.